1. Field of the Invention
The present invention relates generally to a traction control system for an automotive vehicle for providing optimum wheel traction of vehicular wheels. More specifically, the invention relates to a technology for controlling vehicular brake systems for recovering wheel traction, in which operation of the braking system is controlled depending upon the friction level of a road surface.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-85248 discloses a traction control system for an automotive vehicle. The disclosed traction control system is cooperated with a vehicular brake system for applying a braking force for the road wheel for deceleration of the wheel and whereby recovering wheel traction. In such prior proposed traction control system, braking force is established and released depending upon the wheel slippage and variation speed of the wheels slippage. The brake force building up rate cannot be adjusted and thus is increased at a fixed rate. However, the effect of the braking force can be variable depending upon the road surface friction level. Namely, when the vehicle is on a relatively low friction road, the braking speed tends to be low to cause delay in recovery of the wheel traction when the fixed brake pressure building up rate is determined for optimization of the braking effect on a relatively high road surface friction condition. On the other hand, if the fixed rate is relatively high for high braking effect, braking force becomes excessive to cause excessive deceleration of the wheel for degradation of vehicular drivability. Furthermore, a relatively high braking force building up rate may cause excess force in a high friction road for excess deceleration for the wheel for degradation of the vehicle drivability.